One of the most annoying things about YouTube, other than YouTube, is their auto-play feature. That is when one video finishes playing, another starts palying after. There use to be a switch by the Up next section, but that is long gone. At least it doesn't show up on my screen.
A bit of digging online and I found out how to turn it off. Just go to YouTube and paly a video (you don't have to play it full). Move your mouse over the video towards the bottom right-hand corner, where the controls pop up, and the first button you see is the auto-play. Just click on it to disable or enable.
This works if you sign into Google or not.